Method for characterising the knock-resistance of fuels

ABSTRACT

The invention relates to a method for characterising the knock-resistance of a fuel using a test engine, wherein the curve against time of the cylinder pressure of the test engine during combustion of the fuel in the test engine is determined and the determined pressure signals are compared with the corresponding pressure signals of a least one standard fuel of known knock-resistance.

The invention refers to a method for characterizing the knock resistanceof fuels with the help of a test engine.

Standard DIN EN 228 stipulates the characteristic values and propertiesfor non-leaded types of gasoline as minimum requirements. Table 1 showsan extract of the essential characteristic values for fuel.

Requirements according to DIN EN 228 Characteristic value Unit SuperPlusSuper Normal Density at 15° C. Kg/m³ 720-775 Knock resistance R.O.N.min. 98 min. 95 min. 91   M.O.N. min. 88 min. 85 min. 82.5 Lead contentmg/L max. 5   Distillation range * % (V/V) Evaporated quantity (Class A)at 70° C., E70 20-48 at 100° C., E100 46-71 at 150° C., E150   min. 75Evaporated quantity (Class D/D1) at 70° C., E70 22-50 at 100° C., E10046-71 at 150° C., E150   min. 75 Final boiling point (FBP) (ClassA/D/D1) ° C.  max. 210 Volatility indicator VLI ** (VLI = 10 × VP +7/E70) Class D1 Index   max. 1150 Distillation residue % (V/V) max. 2Vapor pressure (DVPE) kPa Class A 45.0-60.0 Class D/D1 60.0-90.0Evaporation residue mg/100 mL max. 5 Benzene content % (V/V) max. 1Sulphur content mg/kg  max. 150 Oxidation stability min  max. 360 Coppercorrosion Extent of max. 1 corrosion * Class A: May 1-September 30(summer) Class D: November 16-March 15 (winter) Class D1: March 16-April30 & October 1-November 15 (transition) ** Vapor Lock IndexEspecially important in all of this is knock resistance, which isdescribed with two characteristic numbers, the motor octane number(M.O.N.) and the research octane number (R.O.N.). Briefly explained, itcan be said that knocking combustion can occur in any gasoline engineand cause extensive engine damage if intensive enough. For this reason,engine developers are required to prevent the non-uniform combustionoccurrence of the knocking—in other words, to integrate knocking controlsystems in engine controls and to constructively prepare the engine forthe fuel's knock resistance. High octane numbers permit higherperformance with the simultaneous higher degree of effectiveness of theengine and therefore lower consumption. For these reasons, higher pricescan also be fetched with higher-octane fuels even tough they have almostthe same energy content (fuel value).

The worldwide determination of octane numbers is nowadays carried outempirically and according to standardized processes in the fuelproducers' laboratories. Special one-cylinder test engines with acompression ratio as variable that can be adjusted to the respectivefuel quality are used for this purpose. The objective is to compare theknocking intensity of the fuel to be tested with fuels of known octanenumber and to determine its number, if needed, by interpolating theoctane numbers. The standard arbitrarily assigned octane number forisooctane is 100 and for n-heptane is 0. By mixing these components, afuel can be produced that will have the same knocking intensity as thefuel to be tested. The octane number that is searched for will thencorrespond to the volumetric share of isooctane in the fuel mixture.Testing conditions differentiate between M.O.N. and R.O.N. if all otherprocess steps agree and the same measuring technique and test enginewere used.

The degree of knocking intensity is generated with an electric sensor(electronic detonation meter) screwed into the engine's combustionchamber (FIG. 1) and an indicator (knock meter) displays the result. Theknocking intensity and frequency calculations are not performed with themeasuring data, however. Our own research has shown us that fuels withequal octane numbers can actually have a different knocking behaviorregarding intensity and frequency. If this method is professionallyapplied with the corresponding experience, an accuracy of no more than+0.2 octane numbers can be achieved. The operation is done manually andtakes between 20 and 30 minutes per octane number, There have been—andstill are—many attempts to determine the octane number with calculationsor another instrument—i.e., outside the engine. Unfortunately, so far ithas not been possible to achieve this with the desired accuracy. If thefuel composition is known, a gas chromatography analysis or infraredspectroscopy can give reasonably accurate results, but these methods areonly used in refineries because they know exactly the composition oftheir fuel. A fundamental improvement of the device and method has notbeen found.

The following problems have been detected in assessing the validprocesses for determining the octane number;

-   -   The accuracy of the process can be improved upon    -   The process is time-consuming; it cannot be automated and allows        no online indication.    -   In the process, the knocking intensity is indicated directly        after the analog processing of the measuring signal. No exact        calculation of the knocking intensity and frequency takes place.    -   Whether the determined octane numbers actually reflect the        knocking resistance of the fuels that current combustion engines        require is called into question.

It is therefore the task of the invention to suggest a process that willmake a fast and reliable characterization of the knocking resistance offuels possible.

The task is solved by determining the chronological sequence of thetesting engine's cylinder pressure during the combustion of the fuel inthe test engine so the determined pressure signals can be compared withthe corresponding pressure signals of at least one standard fuel ofknown knocking resistance.

The utilization of modern measuring and analytical techniques allows theexact characterization of the various parameters of the knockingcombustion processes that occur in the test engine, such as knockingintensity, knocking pressure amplitude of the peak pressure, speed ofpressure increase, frequency and the frequency distributions of theseparameters. The approach taken and the necessary measuring techniquesare described below.

In this case, a piezoelectric pressure sensor (as typically used byengine developers in their R&D tasks) is advantageously built in insteadof the electronic detonation meter. Thus, the signal is amplified andconverted to a voltage signal proportionate to the cylinder pressure (p)(see FIG. 3). The voltage signal is fed to a fast data capture unit,where it is digitalized, processed further and stored.

In this context, it can be advantageous if the pressure signals arefiltered with the help of a band pass filter (especially within the 3 to15 kHz range and/or with high pass from 3 kHz) before comparing themwith the corresponding signals of one or several standard fuels.

At the same time, it is also advantageous if the chronological sequenceof the crank angle (α in FIG. 3) of the test engine is determined duringcombustion with the help of a crank angle sensor. With it, thedetermined pressure signals can be analyzed either as a function of timeor of the crank angle—and with it depending on the ignition timing, forexample. FIG. 4 shows a representation of the cylinder pressure and therespective, time-dependent knocking pressure amplitudes and therefore ofthe crank angle as well.

Knocking intensity or the other parameters mentioned above that can bederived from the cylinder pressure sequence and their distribution as ahistogram, cumulative frequency or individual characteristic number canbe displayed for describing the knocking (FIG. 5). Even a comparison ofthe individual parameters is possible, as important knowledge about theknocking behavior of the examined fuel can be gained. In this respect,it must be pointed out that the respective operating conditions of thetested engine must be considered when interpreting the captured data toprevent false resulting parameters.

A comparison of the corresponding results with those from typicalstandard fuels such as isooctane and/or n-heptane obtained in an analogway leads to one or several characteristic numbers that describe exactlythe knocking resistance of the fuel to be tested.

In this case, the respective characteristic numbers based on thechronological sequence of the test engine's cylinder pressure signalthat are obtained can be measured either during the individual testengine cycle or obtained from the statistical evaluation of the pressuresignals of several cycles.

In addition, it is also advantageous if the compression ratio of thetest engine is calculated based on the cylinder pressure in a definedcrank angle, whereby the crank angle serves as measure of the ignitiontiming. Since the knocking behavior of a fuel depends, among otherthings, from the test engine's compression ratio, it is advantageous totake this parameter into account when comparing the pressure signals ofthe fuel tested with those of the standard fuel.

Furthermore, it is also extremely advantageous if the measured cylinderpressure signals and the corresponding additional known data from thetested fuel—such as, for example, its exact chemical composition—can bestored in a database. After all, these measured values are the basis forfuture evaluations of new or not yet analyzed fuels. In this case (andespecially while complying with standardized process steps), fast andreliable conclusions about the knocking behavior of a fuel to be testedcan be obtained. With the corresponding data stock, an analysis ofstandard fuels is therefore no longer necessary in every case.

Examples of the process steps are listed below:

-   -   Time-based measurement of the cylinder pressure signal (scanning        frequency: 100 kHz)    -   Band pass filtration of the measured pressure signal (3 to 15        kHz)    -   Determination of the maximum amplitude height of the filtered        pressure signal per cycle (best results with a signal processor)    -   Evaluation of the amplitude height and its frequency in an        adjustable time window (300 cycles, intensity distribution or        cumulative frequency)    -   Correlation of results with the known standard fuels or their        mixtures    -   Display and storage of the correlation indices as measure for        knock resistance

The important advantages of the device and the process include:

-   -   Higher accuracy, especially through the use of precision        technology and statistical analysis    -   More exact evaluation of the knocking processes through the        analysis of knocking intensity and frequency    -   Possibility of automation    -   Online display of the characteristic number of the knocking        resistance    -   Automatic or half-automatic process    -   Measurement with standard fuels no longer needed as soon as the        results have been made available in a database    -   The compression ratio can be exactly calculated from the        pressure sequence occurring before the time of ignition

Modifications of the invention are easily possible within the frameworkof the patent claims, in which case it is expressly mentioned that allindividual characteristics published in the patent claims, in thedescription and in the figures can become reality in any combinationthereof as far as this is possible and makes sense. Thus, it can bequite advantageous if the pressure and/or temperature of the combustionmixture and its dwelling time (especially in form of a characteristicnumber calculated from this) can be taken into account. It can also beadvantageous if the ignition timing and the start of knocking of thecombustion mixture can be recorded (especially with the help of asensor) and the intermediate time difference and/or the difference ofthe respective crank angles of the test engine are considered. In thisway, it is possible to determine very accurately the time period to beanalyzed, which results in an especially reliable implementation of thetest. In addition, it is also possible to determine the burning period(heat input through burning) of an individual or several test enginecycles for obtaining there from the ignition delay, the preliminaryignition, the maximum burning speed and/or the residual amount of thecombustion mixture from fuel and combustion gas, especially combustionair, when the knocking starts. The burning period of an individual orseveral cycles of the test engine can also be determined (and at least apre-defined value of the burning period such as the start of burning)with the help of at least one sensor, especially one for measuring theionic current inside the test engine, a sensor for measuring thestructure-borne noise of the test engine and/or an optical sensor. Thismethod allows the very precise determination of the start of the burningso that the period for the corresponding cylinder pressure measurementcan be determined with pretty good reliability. It is likewise extremelyadvantageous for the characterization of knocking resistance to includea statistical analysis of the readings (especially those of the pressuresignals), in which case this statistical analysis should encompass therecording of the readings (especially of the pressure signals) of one orseveral cycles (between 200 and 500, for example), of the test engine,particularly in a defined operating point of the test engine. It wouldalso be advantageous for the statistical analysis to encompass thecalculation of the mean values of the readings so that possiblemeasurement fluctuations should only have a minimal effect on thecharacterization of knocking resistance,

1. Process for characterizing the knocking resistance of a fuel with thehelp of a test engine, characterized in that the chronological sequenceof the test engine cylinder pressure is determined while the fuel burnsin the test engine and the determined pressure signals are compared withthe corresponding pressure signals of at least one standard fuel ofknown knocking resistance. 2-27. (canceled)